FIG. 4 shows a conceptual view of a light waveguide according to a prior art semiconductor laser. In FIG. 4, reference numeral 1 designates an active region. Reference numerals 2 and 2a designate p and n type cladding layers, respectively, between which the active region 1 is sandwiched. Reference numerals 3 designate low refractive index regions which are disposed between cladding layers 2 and 2a on the left and right sides of the active region 1. A pair of resonator facets 4 and 4a are provided at the both end surfaces of the laser resonator. P side and n side electrodes 5 and 5a are disposed on the p type and the n type cladding layers 2 and 2a, respectively. Reference character W represents width of the active region 1.
FIG. 5 shows a refractive index distribution in the direction across the face of the active region 1, that is, in the transverse direction. Reference character N represents the refractive index of the active region 1 and reference character .DELTA.N represents the refractive index difference between the refractive indices of the active region 1 and the low refractive index region 3. This refractive index distribution is the same throughout the resonator length.
The device operates as follows.
When a voltage is applied between the p side electrode 5 and the n side electrode 5a, electrons and holes injected therefrom recombine the active region 1, thereby generating light having a wavelength corresponding to the energy band gap of the active region 1.
This light is confined in the active region 1 because the refractive index N of the active region 1 is higher than that at the periphery as shown in FIG. 5. Accordingly, when the injected current reaches a predetermined value, laser oscillation occurs.
The maximum light output of the semiconductor laser is restricted by the energy density of the laser light at the resonator facets 4 and 4a. In order to obtain a high power output, the width W of the active region 1 is made larger to reduce the energy density.
In the prior art semiconductor laser of such a construction, it is possible to increase the light output by increasing the width W of the active region 1 without changing the refractive index difference .DELTA.N because the refractive index distribution throughout the resonator direction is uniform. When W is increased, however, higher order transverse modes are excited and light including those modes is emitted. Furthermore, if the refractive index difference .DELTA.N is reduced in order to stabilize the mode, the light confinement is weakened and the threshold current is increased.